The present invention relates to an improved apparatus and method for recovering sulfur from gases containing hydrogen sulfide. More particularly, it relates to a method and apparatus which provides increased sulfur recovery and process flexibility and is for use in situations where admixtures such as ammonia, cyanic acid, hydrocarbons or other materials capable of interfering with certain steps of the process are present in the gas containing hydrogen sulfide. In addition, the method and apparatus of the present invention is useful when an increase in the capacity of the process is required.
The invention relates to a general process, known as the Claus process, which is used for sulfur recovery from gases containing hydrogen sulfide (H.sub.2 S). Such gases are often generated by petroleum refineries, natural gas processing plants, coke plants, ore processing operations and other industries. At refineries these gases are commonly classified into two groups: acid gases which are usually richer in H.sub.2 S, and sour gases which contain H.sub.2 S and ammonia, (NH.sub.3). Both gases may also contain CO.sub.2, H.sub.2 O and lesser quantities of hydrocarbons and other admixtures
Depending on the H.sub.2 S content, feed gases are usually classified as rich, lean or very lean. The approximate ranges of concentrations used in this classification are presented in Table 1, below, which also indicates the most typical process modifications that are used to treat feed gases with various H.sub.2 S concentrations. The composition and flow rates of feed gases fluctuate widely over long time periods, and fluctuate dynamically due to changes in raw materials, technology and operation of the processes where the feed gases are generated.
TABLE 1 ______________________________________ Typical Ranges of H.sub.2 S in Feed Gases and the Corresponding Claus Process Modification: H.sub.2 S in Feed Gas (Mole Percent) Modification ______________________________________ 50-100 (Rich Feed) Straight through 20-50 (Lean Feed) Split flow 10-20 (Lean Feed) Split flow with preheating of the feed streams &lt;-10 (Very Lean Feed) Sulfur recycle ______________________________________
The straight through and split flow modifications of the Claus system were originally intended for treatment of clean acid gases composed of H.sub.2 S, CO.sub.2 and H.sub.2 O. A typical straight through Claus system includes a furnace for partial combustion of hydrogen sulfide, a waste heat boiler, a sulfur condenser, several consecutive reheater/catalytic converter/condenser units, a tail gas treatment system and an exhaust stack. The entire hydrogen sulfide containing gas, as well as combustion air, is fed into the Claus furnace burner wherein one third of the hydrogen sulfide is combusted to form sulfur dioxide and water (Equation 1 below). Sulfur dioxide and hydrogen sulfide further react with each other in the reaction chamber of the Claus furnace to form sulfur and water (Equation 2). Reaction (2) is reversible and results in a temperature and pressure dependent equilibrium wherein the gas simultaneously contains SO.sub.2, H.sub.2 S, elementary sulfur and water. These hot gases are exhausted from the reaction chamber into the boiler and the following condenser where they are rapidly quenched. Elementary sulfur is condensed at temperatures greater than the temperatures of water condensation and sulfur crystallization. EQU H.sub.2 S+1.50.sub.2 =SO.sub.2 +H.sub.2 O (1) EQU SO.sub.2 +2H.sub.2 S=3S+2H.sub.2 O (2)
The temperature dependent equilibrium which is established between the species presented in Eq. (2) influences the sulfur conversion in the thermal and catalytic process steps. The equilibrium between SO.sub.2, H.sub.2 S, S and H.sub.2 O in the Claus process is effected by admixtures to the feed gases, nitrogen in the combustion air and products of side reactions such as H.sub.2 and CO. Considering all operable effects, the recovery of sulfur after the boiler and the first stage condenser typically ranges from 50% to 70%. The additional sulfur is recovered in the condensers of the subsequent reheating, catalytic conversion and condensation units. The temperature in the Claus furnace is usually suggested to be in the range from 1040.degree. C. (1900.degree. F.) to 1370.degree. C. (2500.degree. F.). The sulfur recovery across the entire Claus process train may be from 80% to 98%. Unreacted hydrogen sulfide and sulfur dioxide, water vapors and other admixtures are transferred from the Claus process to the tail gas treatment system where the sulfur containing constituents are further removed before the gas is discharges to the atmosphere.
When lean feed gases are treated in the conventional Claus process, the desirable furnace temperature cannot be reached in the straight through process modification utilizing air for combustion. Therefore, split flow modification is used wherein a fraction of the feed gases (usually about 2/3 of the total flow) by-passes the Claus furnace so that the furnace temperature is raised due to the near stoichiometric combustion conditions produced. Further preheating of the feed gases may become necessary to achieve the desired furnace temperature at low H.sub.2 S concentrations. In treating very lean feed gases, a fraction of sulfur may be recycled and used as a fuel in the Claus furnace while a substantial fraction or all of the feed gas by-passes the Claus furnace.
Recent modifications to the Claus process have been developed because many refineries and gas plants are using raw materials with much higher sulfur content than in past years. This increase in the total quantity of acid gases necessitates a capacity increase in the sulfur recovery in the existing Claus process. In addition, more stringent environmental regulations have required modifications to the Claus process to allow the treatment of several waste streams previously discharged to the environment and have required a greater efficiency of sulfur recovery. Additional modifications to the Claus furnace have been developed to allow the processing of waste streams such as sour gases that contain ammonia and other admixtures such as cyanic acid and hydrocarbons.
The capacity of the existing Claus systems cannot be increased by simply feeding more acid and/or sour gases and air. This increase in flow rates causes hydraulic losses across the system to increase. The pressure in the system also increases, thereby posing a danger of gas leakage through the sulfur legs which are used to discharge liquid sulfur from condensers. Simultaneously, the residence time of gases in all units of the system decreases with a respective drop in the system performance. The capacity of existing condensers, reheat units and converters may also be inadequate to receive additional heat load.
B. G. Goar, in U.S. Pat. No. 4,552,747, reviewed the use of oxygen instead of air in the conventional Claus process to increase system capacity without increasing the head losses and without decreasing the residence time. This is achieved by substituting quantities of nitrogen in the combustion air by feed gas and oxygen. In order to minimize furnace overheating due to removal of nitrogen from combustion gases, Goar suggested recycling a 5% to 60% fraction of the gases after the first condenser back into the reaction furnace zone in the Claus process. This recycle gas, at about 180.degree. C. (350.degree. F.), absorbs excessive heat generated in the furnace. The process is claimed to be applicable when the hydrogen sulfide content of the feed gas ranges from 60 to 100 mol% and when the recommended temperature of the reaction furnace zone is in the range of 1315.degree. C. (2400.degree. F.) to 1540.degree. C. (2800.degree. F.).
D. K. Beavon, in U.S. Pat. No. 4,279,882, reviewed problems associated with NH.sub.3 in sour feed gases. Often flows of sour gases are small as compared to acid gases and can be added to the process without any substantial increase in flow rates and hydraulic losses. However, ammonia present in the gases tends to form solid substances while reacting with sulfur containing species, particularly with sulfur trioxide. Therefore, ammonia must be substantially destroyed in the Claus furnace prior to the boiler.
Beavon suggests that ammonia can be efficiently destroyed in a split flow process with two high temperature zones. The first zone is a combustion zone in the burner means fed with sour gas, with 33% to 66% acid gas and with all oxidizing air required in the process. This shifts the ratio of oxygen to combustible constituents in the combustion zone towards stoichiometric and results in higher temperatures. The remaining 67% to 34% of the acid gas by-passes the first zone and is fed into the second zone located in the adiabatic reaction chamber. The hot combustion products of the first zone are also transferred into the second zone wherein no additional oxygen is available for combustion.
A major problem with the Claus process, even with the above described modifications, is that each process modification has been developed for particular operating conditions, e.g. particular gas compositions and flow rates, and lacks adaptability to changing conditions. Accordingly, the wide range variations in the composition and flow rates of the feed gases, as well as substantial dynamic variations in composition and flow rates which occur at many plants, cannot be met by any known modifications of the process. Additionally, each process modification has certain limitations and disadvantages which preclude the most beneficial and reliable use of the Claus process. These limitations and disadvantages are related to the process thermodynamics; to the specifics of the combustion step such as mixing, turbulence, and reaction kinetics; to the specific properties of the feed gases and to the design features of the thermal and catalytic units of the Claus process. These limitations and disadvantages are subsequently discussed.
Shchurin P. M., Pliner V. M. and Nemirovskii M. S. (Gas Industry, No. 6, 1983, Russian) studied the thermodynamics of the thermal step of the Claus process and found that a temperature exists at which the maximum conversion of sulfur occurs. Thermodynamic analyses performed for feed gases of various compositions considering the major and minor constituents formed in the Claus process have demonstrated that the typical curves indicating sulfur conversion versus temperature have distinct minima which separate the left and right branches of each curve. Our calculations presented in FIG. 5 demonstrate the minimum and maximum of the curve. The left and right branches correspond to the predominant S.sub.8 and S.sub.2 formation and the right branches of these curves have maxima. The downward trend of the right branches at higher temperatures is mainly attributable to the dissociation of water, carbon dioxide and other constituents in the reacting gas mixture, which reduces sulfur conversion.
In Claus systems, the equilibria in accordance with the left branch occurs in the catalytic steps while the right branch equilibria occurs in the thermal conversion step. The maxima on the right branches correspond to the optimal temperatures in the thermal conversion step. The thermodynamic computations also demonstrate that the optimal temperatures in the thermal process steps can be obtained by adjusting the amount of nitrogen introduced in the furnace with the oxidizing gas by providing the optimum oxygen to nitrogen ratio. The optimum oxygen to nitrogen ratio can be provided when both oxygen and air is used to form the oxidizing gas and when the oxygen to air ratio can be dynamically controlled.
By virtue of their instrinsic properties, flame reactors never provide ideally uniform distribution of temperatures, concentrations and turbulence. These nonuniformities cause local deviations from the average thermodynamic equilibria. Additionally, the formation of the sulfur product in the Claus process depends on the heat transfer, on transport phenomena such as turbulence and molecular diffusion, on the kinetics of the chemical reactions and on the specific features of the system.
Destruction of ammonia occurs mainly through thermal dissociation and oxidation according to the following formulas: EQU 2NH.sub.3 =N.sub.2 +3H.sub.2 ( 3) EQU 2NH.sub.3 +1.5 O.sub.2 =N.sub.2 +3H.sub.2 O (4)
Due to nonuniform distributions of temperature, concentrations and gas velocities in the Claus furnace, the fraction of ammonia which breaks through the colder zones or zones with a local deficiency of oxygen will not be sufficiently destroyed. Hydrocarbons in feed gases are oxidized by oxygen as well as by water at high temperatures. The fraction of hydrocarbons breaking through the colder zones having a local deficiency of oxygen will also not be sufficiently destroyed. Diluting gases, such as nitrogen in combustion air and the nitrogen, water and carbon dioxide in the recycle gas of the Goar process, reduce the concentrations of NH.sub.3 and O.sub.2 in the reaction mixtures, shift the equilibrium of reactions (3) and (4) to the left and reduce the process rate. Similarly, the degree and the rate of oxidation of hydrocarbons may be reduced.
Experience has proven that neither of the conventional Claus process modifications ensures the required ammonia destruction. Moreover, in split flow processes ammonia, hydrocarbons and other admixtures present in the fraction of the flow by-passing the furnace do not undergo the required thermal destruction. None of the existing modifications to the Claus furnace provide for significant destruction of ammonia in lean gases.
The Goar process modification is intended to increase the throughput of systems used for testing only rich feed gases. In this process the recycle gas laden with increased quantity of water dilutes the combustion products and tends to suppress sulfur conversion. Since the recycle rate is selected based upon the temperatures allowable by refractories, the optimal for sulfur recovery furnace temperature is not provided in the Goar process. Moreover, the heat loading rates on the furnace, boiler, first condenser, first catalytic reactor and second condenser are excessively high which necessitates substantial modifications of these units in order to accommodate for the increased throughput capacity of the system.
The complete destruction of ammonia does not occur in the full scale operation of the conventional Claus process due to low combustion temperatures, imperfect mixing and flow patterns in the combustion apparatus, nonuniform temperature distribution and the presence of water droplets in the feed gases.
Breakthroughs of a fraction of ammonia and other admixtures in the process of the Beavon patent may occur due to poor mixing of the feed gases in the first thermal zone, and due to the absence of the flame in the second thermal zone. Admixtures escaping the first zone and those introduced with the balance of the acid gas fed into the second zone are not subject to the flame reactions in the second zone and may be transferred further along the process train. Ammonia which escapes from the thermal process steps and sulfur species found in the furnace react to form solid salts capable of plugging the catalyst.
Additionally, the tail gases of the Claus process contain CO.sub.2, H.sub.2 O, N.sub.2 and traces of H.sub.2 S and/or SO.sub.2. After tail gas treatment, this gas stream may be used as a valuable source of CO.sub.2 which can be separated and used for various industrial applications. The water can also be condensed and reused. Moreover, if N.sub.2 is not present in the tail gas a zero discharge system can be developed.
There exists a need, therefore, for a method and apparatus which provides greater recovery of sulfur from gases containing hydrogen sulfide and also allows one to increase the throughput of systems of a given size without substantial modification or existing thermal and catalytic stages.
There exists a further need for such a method and apparatus which is adaptable to long term changes in operating conditions and which provides sufficient flexibility to meet short and long term dynamic variations in the composition and flow rate of the feed gases.
There exists yet a further need for such a method and apparatus which provides efficient ammonia destruction in the feed gases by insuring proper temperature, time and turbulence conditions for ammonia contaminated feed gases in the Claus furnace.
There is also a need to eliminate, if possible, the gaseous emission in the atmosphere with simultaneous recovery of CO.sub.2 and possible recycling of water.